Safer Design by Tube Rupture Analysis

Abstract

When a tube ruptures in a shell and tube heat exchanger, the effect of liquid hammering may induce very high transient pressure on shell side due to the leaked mass from tube side travelling to shell side. This article describes a novel technical approach to adequately translate the volume displacement effect by the leaked mass from tube side onto the shell side holdup volume in the unit. The transient pressure from the liquid hammering effect is then accurately predicted by a first principle simulator, and proper mitigation measures may be identified to meet safety requirement while minimizing capital cost. While assuming tube side pressure at tube sheet location remains constant, the mass flow rate profile through the ruptured tube as function of downstream (shell side) local pressure is determined according to industry standards and/or project standards. This profile is then transformed to volumetric flow rate profile displacing shell side hold up volume as function of time in milliseconds time scale. The resulting volumetric profile is then applied to a first principle simulator to predict the transient pressure as a result of liquid hammering effect. The mitigation measure, if any, may be at the same time tested and refined by the simulator. The constraints imposed by the project are iteratively evaluated, and adjusted if necessary, to achieve the best reconciliation among factors of capital cost, safety requirement and project schedule etc. In this article, a compressor discharge after cooler of double shells, with one stacked on top of another, is used for the discussion. Furthermore, the scope of the model extends to include the surrounding piping, and include any considerable lead line length to the relief device. The details of the exchanger geometry, including internal components such as the baffles, bundle type, nozzle etc. are modeled with adequate resolution. The pressure wave propagation along the path of shell side flow in milliseconds time scale are simulated and the localized peak pressures are reported. The high peak pressure necessitates a mitigation measure to be implemented, while maintaining the proposed shell side design pressure to stay for this particular unit. Note that this type of study, for safety concerns, it could result in elevated shell side design pressure, even after considering mitigation measure, leading to major changes to associated supply and return piping, resulting in cost and schedule delays. The technical approach illustrated in this article describes the work flow to transform the mapping of mass flow rate as a function of pressure to volumetric flow rate as a function of time in milliseconds time scale, a technique considered as the first time to be introduced into the practice. The approach increases the fidelity of the study greatly, resulting in reduced capital cost as much as possible, while largely mitigating safety concerns. The approach also affords us to test multiple configurations of pipe size, pipe routing, relief device response, and shell layouts iteratively in a relatively short period of time to optimize the design.